PHOTOS: Tahrir Square violence

Security forces fired tear gas and clashed Monday with several thousand protesters in Tahrir Square in Cairo, Egypt, in the third straight day of violence that has killed at least 24 people and has turned into the most sustained challenge yet to the rule of Egypt's military.

Young activists demanding the military hand over power to a civilian government skirmished with black-clad police, hurling stones and firebombs and throwing back the tear gas canisters being fired by police into the square, which was the epicenter of the protest movement that ousted President Hosni Mubarak in February.